From f8923b1a3fd317a7ed2919cd74539915b361c8b6 Mon Sep 17 00:00:00 2001
From: Jiri Pittner <jiri@pittnerovi.com>
Date: Wed, 11 Feb 2026 16:56:12 +0100
Subject: [PATCH] svd_solve implemented

---
 nonclass.cc |  3 ---
 nonclass.h  | 39 ++++++++++++++++++++++++++++++++++++++-
 t.cc        | 24 +++++++++++++++++++++++-
 3 files changed, 61 insertions(+), 5 deletions(-)

diff --git a/nonclass.cc b/nonclass.cc
index 0981e94..e6b4e98 100644
--- a/nonclass.cc
+++ b/nonclass.cc
@@ -918,9 +918,6 @@ void singular_decomposition(NRMat<std::complex<double> > &a, NRMat<std::complex<
 }
 
 
-
-
-
 //QR decomposition
 //extern "C" void FORNAME(dgeqrf)(const int *M, const int *N, double *A, const int *LDA, double *TAU, double *WORK, int *LWORK, int *INFO);
 
diff --git a/nonclass.h b/nonclass.h
index be198ac..5232e97 100644
--- a/nonclass.h
+++ b/nonclass.h
@@ -158,13 +158,50 @@ extern void linear_solve(NRMat<T> &a, NRVec<T> &b, double *det=0, int n=0); \
 extern void linear_solve(NRSMat<T> &a, NRVec<T> &b, double *det=0, int n=0); \
 extern void diagonalize(NRMat<T> &a, NRVec<LA_traits<T>::normtype> &w, const bool eivec=1, const bool corder=1, int n=0, NRMat<T> *b=NULL, const int itype=1); \
 extern void diagonalize(NRSMat<T> &a, NRVec<LA_traits<T>::normtype> &w, NRMat<T> *v, const bool corder=1, int n=0, NRSMat<T> *b=NULL, const int itype=1);\
-extern void singular_decomposition(NRMat<T> &a, NRMat<T> *u, NRVec<LA_traits<T>::normtype> &s, NRMat<T> *v, const bool vnotdagger=0, int m=0, int n=0);
+extern void singular_decomposition(NRMat<T> &a, NRMat<T> *u, NRVec<LA_traits<T>::normtype> &s, NRMat<T> *v, const bool vnotdagger=0, int m=0, int n=0); \
 
 /*NOTE!!! all versions of diagonalize DESTROY A and generalized diagonalize also B matrix */
 
 declare_la(double)
 declare_la(std::complex<double>)
 
+
+
+//svd_solve without contamination from nullspace, b is n x nrhs, result returns in b, A is destroyed
+template<typename T>
+NRVec<T> svd_solve(NRMat<T> &a, const NRVec<T> &b, double thres=1e-12)
+{
+if(b.size()!=a.nrows()) laerror("size mismatch in svd_solve");
+NRVec<double> w(a.ncols());
+NRMat<T> u(a.nrows(),a.ncols());
+NRMat<T> vt(a.ncols(),a.ncols());
+singular_decomposition(a,&u,w,&vt,false);
+NRVec<T> utb = b*u;
+for(int i=0; i<w.size(); ++i) utb[i] =  w[i]>thres ? utb[i]/w[i] : 0.;
+return utb*vt;
+}
+
+
+template<typename T>
+NRMat<T> svd_solve(NRMat<T> &a, const NRMat<T> &b, double thres=1e-12)
+{
+if(b.nrows()!=a.nrows()) laerror("size mismatch in svd_solve");
+NRVec<double> w(a.ncols());
+NRMat<T> u(a.nrows(),a.ncols());
+NRMat<T> vt(a.ncols(),a.ncols());
+singular_decomposition(a,&u,w,&vt,false);
+NRVec<T> ww(w.size());
+for(int i=0; i<w.size(); ++i) ww[i] =  w[i]>thres ? 1./w[i] : 0.;
+NRMat<T> utb(a.ncols(),b.ncols());
+utb.gemm((T)0,u,'c',b,'n',(T)1);
+utb.diagmultl(ww);
+NRMat<T> res(a.ncols(),b.ncols());
+res.gemm((T)0,vt,'c',utb,'n',(T)1);
+return res;
+}
+
+
+
 // Separate declarations
 //general nonsymmetric matrix and generalized diagonalization
 //corder =0 ... C rows are eigenvectors, =1 ... C columns are eigenvectors
diff --git a/t.cc b/t.cc
index c835e60..9c319a2 100644
--- a/t.cc
+++ b/t.cc
@@ -4665,7 +4665,7 @@ NRMat<double> b = explicit_matrix<double,NRMat<double> >(a);
 cout <<"Error = "<<(a-b).norm()<<endl;
 }
 
-if(1)
+if(0)
 {
 int m;
 int which;
@@ -4696,5 +4696,27 @@ for(int i=0; i<m; ++i)
 
 }
 
+if(0)
+{
+NRMat<double> a;
+NRVec<double> b;
+cin >>a>>b;
+NRMat<double> aa(a);
+NRVec<double> x = svd_solve(aa,b);
+cout <<x;
+cout <<"Error = "<< (a*x-b).norm()<<endl;
+}
+
+if(1)
+{
+NRMat<double> a;
+NRMat<double> b;
+cin >>a>>b;
+NRMat<double> aa(a);
+NRMat<double> x = svd_solve(aa,b);
+cout <<x;
+cout <<"Error = "<< (a*x-b).norm()<<endl;
+}
+
 
 }//main